This invention relates to devices and methods that employ ferrite-type materials to produce heat in an alternating magnetic field. Ferromagnetic materials and ferrites have been used in various systems and devices for heat producing purposes and for non-heat producing purposes. Ferrite powders have been used to produce heat by hysteresis losses and/or skin effect eddy current losses when placed in an electromagnetic field provided by an induction coil powered by an alternating current power source. Ferromagnetic materials have been used in layers to produce heat from skin effect losses when powered by an alternating current.
The use of ferrites and ferromagnetic materials to produce heat by induction heating is illustrated in U.S. Pat. No. 3,391,846 to White et al., wherein antiferromagnetic particles, such as a ferrite powder, are used to produce heat where it is desirable to cause chemical reactions, melt materials, evaporate solvents, produce gasses and for other purposes. In White et al., a material containing the nonconductive antiferromagnetic particles was passed through or near an induction coil thus subjecting them to a high frequency alternating magnetic field of at least 10 MHz, thereby heating the particles to their Neel temperature.
In Japanese Kolsoku Disclosure No. 41-2677 (Application No. 39-21967) a ferrite material is placed inside an induction coil and heated by a high frequency alternating current. Objects, such as fibers, are then passed through openings in the ferrite material to heat treat by conduction the objects at the Curie temperature of the ferrite material.
In co-pending U.S. application Ser. Nos. 07/404,621 filed Sep. 8, 1989, 07/465,933 filed Jan. 16, 1990, and 07/511,746 filed Apr. 20, 1990, all hereby incorporated herein by reference, various devices and methods are disclosed utilizing ferrite powder and similar ferromagnetic or ferrimagnetic materials in the magnetic field of an induction coil to produce improved and effective heating in particular applications. Application Ser. No. 07/404,621 discloses auto-regulating, self-heating recoverable articles which, when subjected to an induction coil alternating magnetic field, heat to the Curie temperature of the particles by induction heating to generate sufficient heat to cause the heat recoverable articles to recover to their original configuration. U.S. application Ser. No. 07/465,933 discloses a system for providing heating in an article or object in an induction coil alternating magnetic field using lossy, heat producing magnetic particles in combination with non-lossy particles which have high permeability and which are not heat producing particles. The non-lossy particles serve to maintain the coupling of the magnetic circuit and maintain the desired magnetic field focus and intensity through the area in which the lossy heat producing particles are positioned. U.S. application Ser. No. 07/511,746 discloses a removable heating article for use in an alternating magnetic field created by an induction coil in which a base material carries lossy heating magnetic particles. The article can be attached to a substrate and removed therefrom after being subjected to the magnetic field created by an induction coil and after the heating is completed.
Ferromagnetic materials have also been used in heating devices that employ the skin effect heating phenomenon to provide self-regulating heating devices. For example, U.S. Pat. Nos. 4,256,945 and 4,701,587, both to Carter and Krumme, disclose a self-regulating heater such as a soldering iron tip, which consists of an outer nonmagnetic shell which is in good thermal and electrical contact with an inner ferromagnetic shell or layer. An inner conductive, nonmagnetic stem extends axially into the assembly formed by the inner and outer shells, and may be joined to the inner shell. A power supply is connected to the stem and the outer shell. A self-regulating soldering iron is achieved by the selection of a ferromagnetic material having a Curie temperature above the melting point of the solder. When high frequency, constant current power is applied between the stem and the outer shell, current flows primarily in the ferromagnetic material and produces heat due to the skin effect resistance losses. When the device approaches Curie temperature, the ferromagnetic material becomes nonmagnetic and the current flows primarily in the copper outer shell. Since the current is constant and the copper has substantially less electrical resistance than the ferromagnetic material, heating is greatly reduced while the ferromagnetic layer is at or above its Curie temperature. As a consequence, the temperature of the device is regulated near the Curie temperature of the ferromagnetic material chosen.
U.S. Pat. No. 4,914,267 to Derbyshire also discloses skin effect type heaters which use ferromagnetic materials having a desired Curie temperature in electrically conductive layers to provide auto-regulated heating to the Curie temperature of the material upon application of an alternating current to the conductive layer of ferromagnetic material. The power applied to the ferromagnetic layer is in the form of an alternating current which produces skin effect current heating in the continuous ferromagnetic layer. As the ferromagnetic layer reaches its Curie temperature, the permeability of the layer drops and the skin depth increases, thereby spreading the current through the wider area of the ferromagnetic layer until the Curie temperature is achieved throughout and the desired heating is achieved. The alternating current is supplied to the ferromagnetic layer either directly from a power source through electrodes in the conductive layer of ferromagnetic material or is supplied inductively from an adjacent insulated conductive layer directly powered with the alternating current. Another type of auto-regulating skin effect type heater is disclosed in U.S. Pat. No. 4,659,912 to Derbyshire in the form of a flexible strap heater which includes a ferromagnetic layer.
In U.S. Pat. No. 4,745,264, Carter discloses a self-regulating heater in which inductive coupling is employed to couple a constant current into a ferromagnetic layer surrounding and contacting a copper rod forming a rearward extension of the tip of the soldering iron. The induction coil employed to couple current into the magnetic material surrounds the layer of conductive ferromagnetic material.
U.S. Pat. No. 4,839,501 to Cowell illustrates another example of such a self-regulating cartridge soldering iron having a replaceable tip. The cartridge includes a helical induction coil wound around a tip extension rod having a layer of high Mu ferromagnetic material.
In U.S. Pat. No. 4,877,944, Cowell et al. disclose another self-regulating heater in which the core is shaped so as to focus the magnetic flux in the layer of ferromagnetic material of the heater. The core may be "I" or "E" shaped in cross-section and has a coil wound about its narrow section(s). Also, it is disclosed that an outer magnetic layer is disposed outside the coil to act as a magnetic shield and restrict spreading to the magnetic flux.
In art areas unrelated to heating devices, ferrimagnetic materials and in particular ferrites in the form of beads, blocks, rings, etc. are conventionally placed on electrical conductors to provide various functions, such as RF/EMI shielding, signal isolation, noise suppression, transient filtering, oscillation damping, high frequency filtering or damping, and the like. However, these prior conventional uses of ferrite bodies do not produce significant heat in the ferrite body. While the filtering or damping function provided by a ferrite body may incidentally convert the filtered signal or frequency to a small amount of heat, the amount of heat produced is insignificant or inconsequential in the device or in the environment where the ferrite body provides the desired filtering or damping function. In fact, it has been recognized in the art that even significant heat, especially excessive heat, is to be avoided in such systems because such heat would unduly heat nearby electrical components and interfere with the function of the circuit or device.
While the heating devices described above are useful and have certain advantages in various applications compared to other devices, they also have certain disadvantages, particularly with respect to other applications. The devices comprising induction coils require high temperature wire insulation with small gauge wire to achieve the small size of the heater device desired for many heater or soldering iron tip applications. Due to the small gauge of the wire, the current capacity is limited, as is the output power of the device. Also, the necessity of having the induction coil present to provide the required magnetic field limits the configurations in which the heater device can be made.
The skin effect, eddy current, layer type heater devices are likewise very effective and have certain advantages in many applications, but have certain disadvantages with respect to certain other applications. For example, the power or current capacity, and the heat producing capacity, are sometimes limited by the capacity of the layers in the device. In addition, these ohmically connected devices are typically low in impedance and require bulky, inefficient and high current capacity impedance matching networks.
In still other art areas also unrelated to heaters, ferrite bodies, such as beads, have also been used as sensors, switches, fuses and controls in various electrical circuits. These uses primarily utilize the Curie temperature effect of a ferrite body. For example, a ferrite bead is placed on a conductor in a particular electrical circuit and the presence of the bead provides a certain impedance and/or resistance in that part of the circuit. When the ambient or surrounding environment temperature raises the temperature of the ferrite body above its Curie temperature, the ferrite body experiences a sharp loss in magnetic permeability. This loss of magnetic permeability by the bead causes a change in the characteristic of the circuit, thus signaling some other part of the circuit that the specified ambient temperature or surrounding environment has been reached.
In the heater device art ferrite bodies have been used as sensor/control elements. An example of such sensor/control use of ferrite bodies in a heated device is illustrated in U.S. Pat. No. 4,849,611 to Whitney et al., which relates to a self-regulating heater. The embodiments disclosed at FIGS. 12c and 19a include a number of ferrite beads strung on a conductive wire (together referred to therein as the reactive component), which is connected in parallel to a resistance heater member or element. When a current is applied, the resistance heating element produces heat, which heats the ferrite beads by conduction, convection and/or radiation. When the ferrite beads are thus heated by the heat generated by the resistance heater element to their Curie temperature, their magnetic permeability sharply decreases. Thus, the reactive component of the circuit containing the ferrite beads is a temperature-responsive sensor part of the circuit. When the magnetic permeability of the ferrite beads drops at their Curie temperature, this allows the reactive component to change the parallel circuit balance so that the current flow through the resistive heating component is decreased. When the device cools so that the ferrite beads cool below their Curie temperature, their magnetic permeability increases, thereby increasing the current flow through the resistance heater element and causing increased heating to again occur in the resistance heater element. This parallel circuit arrangement allows regulation of the temperature of the resistive heater element at the Curie temperature of the adjacent ferrite beads. The ferrite bead elements in that circuit thereby function in their conventional manner to act as temperature sensor/circuit control. In that device the ferrite beads do not produce any significant heat themselves, as evidenced by the parallel circuit arrangement and by the low frequency power supply utilized.
The resistive heating element/reactive-control element type of heater devices have disadvantages associated with the fact that the resistive heating element and the reactive-control element must be in thermal contact or proximity, which restricts the size of the total heating device making it unsuitable for many applications. Also, the temperature of the reactive-control component lags behind the temperature of the heat generating component resulting in undesired temperature oscillation instead of the desired self-regulation at a constant temperature. In addition, thermal resistance between the resistance heater and the ferrite sensor elements is high; because of this the thermal response of the heater to changing thermal loads is poor.
In view of the above, it is apparent that there is a need for improved self-regulating heaters. The present invention has been developed to provide self-regulating heaters and methods for making and using heaters which have various advantages and which do not have the disadvantages mentioned above.
Therefore, it is an object of this invention to provide a self-regulating heater which provides efficient heat generation without the use of layers or skin effect, eddy current heating.
It is a further object of the present invention to provide a self-regulating heater which does not require the presence of a multiple turn, wire coil or an induction coil and associated high temperature electrical insulation for the coil wire.
It is a further object of the present invention to provide a self-regulating heating device that can be made in small sizes having a high watt-density and high power capability.
It is a further object of this invention to provide a self-regulating heater which does not require separate elements or components for heating and for sensing/control to provide self-regulation.
It is a further object of the present invention to provide a self-regulating heater which is inexpensive, easy to manufacture and which can be made in any configuration desired for applying or distributing heat to a desired object or material.
It is a further object of the present invention to provide a self-regulating heater which has an inherent high impedance for easier impedance matching with high frequency, alternating current power sources.
It is a further object of the present invention to provide a self-regulating heater which has a high switching ratio and a quick response time.
The above, as well as other objects, are achieved by the present invention as will be recognized by one skilled in the art from the following summary and description of this invention.